The proposed research is to investigate genetic regulation of tyrosinases in the development of the reproductive system in the laboratory rat, Rattus norvegicus. Preliminary studies have shown tyrosinase activity in tissue extracts from both male and female reproductive organs of immature rats between 20 and 40 days of age. Electrophoresis and assays of activity of the tyrosinases are preliminary to purification and characterization of the enzymes specific for the several reproductive organs. Histochemical studies of the enzymes in the tissues and pedigree studies utilizing enzyme variants are proposed. A correlation of tyrosinases with reproductive organ development will be the first investigated for mamals. Such information may help explain abnormalities in man where there is failure to complete development of the reproductive organs.